The invention relates to a light source with a light-emitting element that emits in a first spectral realm, preferably in the blue and/or ultraviolet range of that visual spectrum, and with a luminophore that either is derived from the group of alkaline-earth ortho-silicates or that contains at least a component from this group of luminescent materials that absorbs a portion of the emission from the light-emitting element and then emits in another region of the spectrum, preferably in the yellow-green, yellow, or orange ranges. The luminophore selected may also be used in mixtures with other luminophores from this group and/or with other luminescent materials that do not belong to this group.
The light-emitting element is preferably an inorganic light-emitting diode (LED), but may also be an organic LED, a laser diode, and inorganic thick-layer electro-luminescence film, or an inorganic thin-layer electro-luminescence component.
Inorganic LED""s distinguish themselves by, among other things, long service life, low space requirements, insensitivity to vibration, and narrow-band spectral emissions.
Numerous emission colors, especially wide-band spectral colors, cannot be realized from LED""s because of the intrinsic emission of an active semiconductor material, or can only be inefficiently realized. This especially applies to the creation of white light.
In accordance with the state of the art, emission colors that cannot be intrinsically realized by a semiconductor are created using color conversion.
The technique of color conversion is essentially based on the principle that at least one luminophore is positioned above the LED xe2x80x9cdie.xe2x80x9d It absorbs a portion of the emission from the die, and is thus excited to photo-luminescence. The emission or light color of the source then results from the mixing of the emission transmitted from the die with the emission emitted from the luminescent material.
Either organic or inorganic systems may basically be used. The significant advantage of inorganic pigments is their higher chemical, thermal, and emission stability in comparison to organic systems. In connection with the long service life of inorganic LED""s, long-life inorganic luminophores ensure a high level of color stability of the light source consisting of both light sources.
If the emitted emission from LED""s emitting blue is to be converted into white light, luminescent materials are used that effectively absorb the blue light (450-490 nm) and convert it into predominantly yellow luminescent emission. However, there is only a limited number of inorganic luminophores that meet these specifications. At this time, materials from the YAG class of luminescent materials are used as color conversion pigments for blue LED""s (WO 98/05078; WO 98/05078; WO 98/12757). These, however, include the disadvantage that they possess a high degree of efficiency only at an emission maximum of less than 560 nm. For this reason, only cold white light colors with color temperatures between 6,000 K and 8,000 K, and accordingly with comparatively reduced color reproduction (typical values for color reproduction index Ra lie between 70 and 75), may be used with the YAG pigments in combination with blue diodes (450-490 nm). This results in severely-limited application possibilities. On the one hand, higher demands are imposed as a rule during application of white-light sources for general illumination, and on the other, consumers in Europe and North America prefer warmer light colors with color temperatures between 2,700 and 5,000 K.
It is further known from WO 00/33389 to use Ba2SiO4:Eu2+ among others as a luminophore to convert the light from blue LED""s. The maximum of the emission from the Ba2 SiO4:Eu2+ luminescent material is, however, 505 nm, so that white light cannot be reliably created using such a combination.
In works by S. H. M. Poort et al.: xe2x80x9cOptical Properties of Eu2+-activated orthosilicates and orthophosphates,xe2x80x9d Journal of Alloys and Compounds 260 (1997), pp. 93-97, the characteristics of Eu-activated Ba2SiO4 and of phosphates such as KBaPO4 and KSrPO4 are investigated. It was also determined here that the emission from Ba2SiO4 is about 505 nm.
The task of this invention is to alter a light source of the type mentioned at the outset so that white light colors with warmer color temperatures, especially those color locations that lie within the tolerance ellipses established by the CIE for general illumination may be created under conditions of high luminous efficiency and a high degree of color reproduction quality.
This task is solved by a light source based on the invention of the type mentioned at the outset so that the luminophore is an alkaline-earth ortho-silicate activated with bivalent Europium of the following composition:
(2-x-y)SrOxc2x7x(Bau, Cav)Oxc2x7(1-a-b-c-d)SiO2xc2x7aP2O5bAl2O3cB2O3dGeO2:yEu2+
where
0xe2x89xa6x less than 1.6 0.005 less than y less than 0.5x+yxe2x89xa61.6
0xe2x89xa6a,b,c,d less than 0.5 u+v=1
applies;
and/or an alkaline-earth ortho-silicate of the following composition:
(2-x-y)BaOxc2x7x(Sru, Cav)Oxc2x7(1-a-b-c-d)SiO2xc2x7aP2O5bAl2O3cB2O3dGeO2:yEu2+
where
0.01 less than x less than 1.6 0.005 less than y less than 0.5
0xe2x89xa6a,b,c,d less than 0.5 u+v=1 xxc2x7uxe2x89xa70.4
applies, whereby preferably at least one of the values a, b, c, and d is greater than 0.01. A portion of the Silicon may be replaced by Gallium in both formulas.
Surprisingly it has been found that white light with good color reproduction and a high degree of luminous efficiency may be realized through a combination of a blue LED with a luminophore selected from a group of alkaline-earth ortho-silicates activated with Europium of the above-named composition based on the invention. In contrast to luminophores based on pure Barium ortho-silicates that emit bluish-green light, yellow-green and yellow to orange luminescent light may be created using Barium-Strontium-orthosilicate mixed crystals, and even completely orange luminescent light may be created by incorporation of Calcium into the ortho-silicate crystal lattice, so that, by mixing the transmitted light from the blue LED with the luminescent light from the selected luminophore, white light with good color reproduction and a high degree of luminous efficiency may be generated. Displacement of emission color by means of substitution of Ba with Sr in ortho-cilicates has previously been known only for excitation using hard UV emission (254-nm excitation) from the above-mentioned work by Poort et al. No description was made of the fact that this effect surprisingly occurs more strongly under irradiation with blue light in the range of 440-475 nm. Baxe2x80x94Srxe2x80x94Ca ortho-silicate mixed crystals and their strong emission capability under excitation with low-frequency UV emission or blue light were previously completely unknown.
The selected luminophore may also be used in mixtures with other luminophores of this group and/or with additional luminescent materials not belonging to this group. The latter luminophores include, for example, blue-emitting alkaline-earth aluminates activated using bivalent Europium and/or Manganese, along with the red-emitting luminophores of the group Y(V,P,Si)O4:Eu,B1, Y2O2S:Eu,Bi, or :Eu2+,Mn2+ alkaline-earth Magnesium di-silicates activated with Europium or Manganese according to the formula
Me(3-x-y)MgSi2O8:xEu, yMn,
whereby
0.005 less than x less than 0.5 0.005 less than y less than 0.5
and Mexe2x95x90Ba and/or Sr and/or Ca applies.
As will be shown in the following embodiment examples, the Sr component in the mixed-crystal luminophores based on the invention must not be too small in order to be able to generate white light.
Surprisingly, it has further been found that the additional inclusion of P2O5, Al2O3, and/or B2O3 into the crystal lattice, as well as the substitution of a portion of the Silicon by Germanium, may also have a significant influence on the emission spectrum of a given luminophore, so that this may be further advantageously varied for a particular application. For this, smaller ions than Si(IV) cause displacement of the emission maximum into a lower-frequency range, while larger ions displace the bulk of the emission into higher frequencies. It could further be shown that it is advantageous for the crystallinity, emission capability, and particularly for the stability of luminophores based on the invention if small amounts of monovalent ions such as halogenides and/or alkali metal ions are additionally included in the luminophore.
Based on a further advantageous embodiment of the invention, the light source includes at least two different luminophores, whereby at least one is an alkaline-earth ortho-silicate luminescent material. The white tone required for a particular application may be especially accurately adjusted in this manner, and Ra values greater than 80 may particularly be achieved. A further advantageous version of the invention consists of a combination of an LED emitting in the ultra-violet range of the spectrum, e.g., in the range between 370 and 390 nm, with at least three luminescent materials, of which at least one is an alkaline-earth ortho-silicate luminescent material based on the invention. Blue-emitting alkaline-earth aluminates activated with Europium and/or manganese and/or red-emitting luminophores from the group Y(V,P,Si)O4:Eu,B1, Y2O2S:Eu,Bi, or from the group of alkaline-earth Magnesium di-silicates activated with Europium and Manganese may be used as additional luminescent materials in corresponding mixtures of luminescent materials.
Several options exist for mechanical implementation of the light source based on the invention. Based on one embodiment example, it is intended that one or more LED chips be positioned on a circuit board within a reflector, and the luminophore be dispersed in a light disk positioned above the reflector.
It is also possible to position one or more LED chips on a circuit board within the reflector, and to mount the luminophore on the reflector.
The LED chips are preferably cast in a domed shape using a transparent casting compound. On the one hand, this casting compound provides mechanical protection, and on the other, it also improves the optical characteristics (better escape of the light from the LED dice).
The luminophore may also be dispersed in a casting compound that connects a configuration of LED chips on a circuit board with a polymer lens, preferably one without gas content, whereby the polymers and the casting compound include refractive indices that vary from one another by no more than 0.1. This casting compound may directly include the LED dice, but it is also possible that they be cast using a transparent casting compound (this results in a transparent casting compound and a casting compound containing the luminophore). Because of the similar refractive indices, there is very little loss at the bordering surfaces due to reflection.
The polymer lens preferably is of spherical or ellipsoid shape that is filled by the casting compound, so that the LED array is secured closely adjacent to the polymer lens. The height of the mechanical structure may thus be reduced.
In order to achieve uniform distribution of the luminophore, it is useful if the luminophore is reduced to slurry in a preferably inorganic matrix.
When using at least two luminophores, it is useful if the minimum two luminophores are individually dispersed within matrices that are positioned one after the other within the spread of light. Thus, the concentration of luminophores may be reduced in comparison to that obtained in a uniform dispersion of various luminophores,
The essential steps to manufacture the luminophore using an advantageous version of the invention are shown in the following:
Depending on the selected composition for production of the alkaline-earth ortho-silicate luminophore, the stoichiometric quantities of alkaline-earth carbonate, Silicon dioxide, and Europium oxide output materials are mixed internally, and are converted into the desired luminophore using the solid-body reaction conventionally used in the production of luminescent materials in reduced atmosphere at temperatures between 1100xc2x0 C. and 1400xc2x0 C. For this, it is advantageous for the crystallinity to add small amounts, preferably smaller than 0.2 mol, of ammonium chloride or other halogens to the reaction mixture. Within the meaning of the displayed invention, a portion of the Silicon may be replaced by Germanium, Boron, Aluminum, or Phosphorus, which may be realized by the addition of corresponding amounts of compounds of the named elements that may be converted thermally into oxides. In a similar manner, it is possible for small amounts of alkali metal ions to be included in the particular lattice.
The ortho-silicate luminophores thus obtained emit at wavelengths between about 510 nm and 600 nm, and possess a half-width value of up to 110 nm.
By means of proper configuration of reaction parameters and specific additives, e.g., of monovalent halogenide and/or alkali metal ions, the distribution of grain sizes of the luminophore based on the invention may be adapted to the demands of the particular application without having to use damaging mechanical size-reduction processes. In this manner, all narrow- and wide-band grain-size distributions with mean grain sizes d50 of about 2 xcexcm and 20 xcexcm may be adjusted.